Which statement about calcium distribution is true?

• A. 40% protein-bound, 10% complexed, 50% ionized
• B. 20% protein-bound, 30% complexed, 50% ionized
• C. 60% protein-bound, 20% complexed, 20% ionized
• D. 10% protein-bound, 40% complexed, 50% ionized

Answer: (A)

Calcium in plasma exists in three main forms: ionized (free) calcium, which is physiologically active; protein-bound calcium, mainly to albumin; and calcium complexed with small anions like citrate or phosphate. In a typical state, about half of the total calcium is ionized, roughly 40% is protein-bound, and about 10% is complexed. This distribution explains why the statement with 40% protein-bound, 10% complexed, and 50% ionized is true. It also highlights why total calcium can change with variations in albumin or pH without necessarily changing the active ionized calcium fraction. Other distributions would imply an unusually high or low share of any form, which doesn’t align with normal serum calcium physiology.